This invention relates to methods and apparatus for fabricating optical waveguide gratings, such as optical fibre gratings, and/or characterising optical waveguides, such as optical fibres.
Optical fibre Bragg gratings are one of the most promising areas of research and development in fibre optic systems. Many systems rely on the precise wavelength selective capability of Bragg gratings such as lasers and sensors and more systems are likely to take advantage of high quality gratings in the near future.
Probably the biggest incursion of the fibre Bragg grating has been in telecommunication systems and especially in dispersion compensation. Chirped fibre gratings are particularly well suited to dispersion compensation as they are compact, exhibit low loss, are highly dispersive and are not subject to the non-linear effects which afflict specialised dispersion shifted and dispersion compensating fibres. Transmission experiments incorporating fibre gratings for dispersion compensation have successfully been demonstrated many times. Potentially the performance of fibre gratings could be further enhanced in dispersion compensation systems with more precise control over the dispersion profile. in particular a reduction in time delay ripples and addition of third order compensation. required for higher bit-rate systems.
FIGS. 7a to 7d of the accompanying drawings illustrate problems which can occur in a nominally linearly chirped fibre grating. FIGS. 7a and 7c illustrate the reflection and time delay characteristics of the grating, and FIGS. 7b and 7d illustrate deviations from the expected characteristics.
This invention provides a method of detecting diameter variations in an optical waveguide, the method comprising the steps of fabricating a chirped grating in the optical waveguide of a known physical pitch; and measuring deviations from the expected time delay of the chirped grating.
The invention also provides a method of fabricating an optical fibre waveguide grating in a waveguide of nominally uniform diameter, the method comprising the repeated steps of: fabricating a grating section in a portion of the optical waveguide of a known physical pitch; measuring deviations from the expected response of the at least the most recently written grating section; and varying a grating parameter for writing a next grating section in dependence on the measured deviations for at least the most recently written grating section.
The invention also provides a method of fabricating an optical grating in a waveguide of nominally uniform diameter, the method comprising the step of varying a grating characteristic at positions along the grating in a substantially inverse relationship to the diameter of the waveguide at those positions.
The invention also provides an optical waveguide grating formed in a waveguide of nominally uniform diameter. in which a grating characteristic is varied at positions along the grating in a substantially inverse relationship to the diameter of the waveguide at those positions.
The invention is based on the new recognition that when fabricating fibre gratings in conventional, i.e. step-index fibre, that the reflection wavelength depends not only on the fibre NA numerical aperture, but also the fibre cut-off. This is because the reflective wavelength, xcexB is given by
2neffxc2x7xcex9B
where neff is the effective fibre index for the guided mode and xcex9B is the actual period of the grating lines.
It has been observed that when drawing fibre, small fluctuations in the fibre diameter generally occur. For a nominal fibre diameter of 125 xcexcm, diameter deviations as large as xc2x11 xcexcm with a period in the range 100-200 mm (along the fibre) have been observed.
A step-index fibre has core index n1 and cladding index n2 (where n1 greater than n2 and NA=n12xe2x88x92n22). The effective index neff depends on the proportion of the guided mode overlapping the core, xcex7 and can be expressed
neff=xcex7n1+(1xe2x88x92xcex7)n2.
The overlap parameter for a given fibre is documented for example in Snyder and Love.
For a typical fibre NA of 0.2, nominal diameter of 125 xcexcm and cut off of 1250 xcexcm a 1 xcexcm diameter change causes a xcx9c50 pm grating wavelength shift. As the diameter varies along the length of a grating, therefore, the response of the grating can deviate significantly from that which might be expected.
In the case of linearly chirped fibre gratings, the effect of these diameter fluctuations is to cause the time delay vs wavelength characteristic to deviate from a linear characteristic.
Within the broad overall aspect of the invention. several preferred techniques are proposed here to allow the fabrication of a desired grating in a non-uniform fibre. The first three techniques involve adjusting the written grating period xcex9B to reduce the impact of the diameter fluctuation on neff. The fourth technique involves UV-pre or post-processing the fibre to adjust n1 or n2 in the region of the core along the fibre such that neff becomes more uniform along the fibre. Obviously any combination of these methods could be employed. All of these methods can be incorporated into current grating fabrication techniques.